The present invention relates to a digital device for correcting registration error, such as misconvergence and raster distortion, in an image display device in which a CRT (cathode-ray tube) is used. Image display devices mentioned here include color television sets and display terminals used in information processing systems.
The invention also relates to a digital registration error correction device for a multi-scan display device which is used for various applications with different scanning line numbers, different deflection frequencies, and different aspect ratios.
Color display devices employing a CRT include display devices of a directly-viewed type and three-tube video projectors. In the former, the three primary colors of light are generated to compose a color image on a fluorescent screen of a CRT. In the latter, lights from three CRTs are passed through color filters and make up an image on a screen. In any case, the three primary colors must be precisely registered with each in order to avoid color fringing. This may be accomplished by the use of a convergence device for correcting misconvergence (color misregistration) by causing correcting waveform currents synchronous with the deflection to flow through auxiliary deflection coils provided at the rear of the deflection coils, called main deflection coils for distinction, of the CRT for making fine adjustment of deflection of each of the electron beams for red, blue and green, independently of each other. The above correcting waveform currents may be generated using analog signals. However, digitally generated ones are used more recently in order to achieve more precise correction.
In a display device using a CRT, electron beams are deflected in vertical and horizontal directions by deflection coils to form a raster consisting of horizontal scanning lines vertically separated from each other.
In a digital convergence device for performing convergence correction of a picture displayed on a screen of a display device using a CRT, correction points are specified in rows and in columns. That, is, the correction points are specified at intersections of selected scanning lines, called correction scanning lines, and imaginary vertical lines horizontally separated from each other. The raster is divided into a plurality of segments by the correction scanning lines, and the scanning lines between the adjacent correction scanning lines are called interpolated scanning lines. The convergence correction for the respective points in the raster is performed using the correction data for the correction points which are stored in a correction data memory, and the interpolated correction data for the interpolated scanning lines. The interpolated correction data is obtained by interpolation on a real-time basis from the correction data of the correction points.
Taking an example of three-tube video projectors, further explanations will be given below. FIG. 43 is a block diagram of a conventional digital convergence device. In order to achieve convergence by shifting the electron beams for the three colors R, G, and B in the horizontal and vertical directions, a total of six channels of correction signals (RH, RV, GH, BH, BV) are needed. FIG. 43 however shows only such a part relating to one channel of them. It should be noted that when this invention is applied to display devices of directly-viewed type, four channels of correction outputs are necessary.
Shown in FIG. 43 are an input terminal 1 to which the horizontal and vertical synchronous signals SA1 synchronized to the deflection, an address generator 212 receiving the synchronous signal SA1 and data on the bus lines 62 of a microprocessor 6 and generating address signals SA2 and interpolated scanning line number signal SCA in synchronism with the deflection, a crosshatch signal generator 3 for generating a crosshatch signal SA4 on the basis of the address signal SA2, a video circuit 4 supplying a video signal for displaying a picture including a crosshatch pattern responsive to the crosshatch signal SA4, and a control key pad 5 for inputting correction positions on the raster and correction values.
The microprocessor 6 mentioned above is for inputting an output signal from the control key pad 5 and writing the correction data indicative of the correction value in the address of a correction data memory 7 corresponding to the position on the raster. The correction data memory will be described later.
The microprocessor 6 is provided with a built-in ROM 61 (non-volatile memory). The correction data memory 7 is in the form of a frame memory and receives, as one input, the address signal SA2 generated by the address generator 202, and data on the bus lines 62 as another input, and outputs the correction data SA6. A vertical interpolator 208 receives correction data SA6 from the correction data memory 7, the address signal SA2 and the interpolated scanning line number data SCA from the address generator 2 and finds correction data of interpolated scanning lines by vertical interpolation.
A D/A (digital-to-analog) converter 9 converts the output of the vertical interpolator 208 into an analog signal. A lowpass filter (LPF) 10 receives the output of the D/A converter 9. An output circuit 11 receives the output of the LPF 10. An auxiliary deflection coil 12 receives the output of the output circuit 11 as an input.
A channel memory 13 is formed of non-volatile memories such as an EEPROM or flash memory and stores the correction data for each of a plurality of display modes. When the display device is used for display of a picture in a selected one of a display mode, the correction data of the particular display mode is transferred to the correction data memory 7, and the vertical interpolation for obtaining the correction data for the interpolated scanning line is conducted repeatedly reading the correction data from the correction data memory 7, as will be later described in further detail.
The crosshatch signal generator 3, the address generator 202, the correction data memory 7 and the channel memory 13 are connected via the bus lines 62 to the microprocessor 6.
Next, we will discuss the procedure of adjustment and the operation of the device during the adjustment. The adjustment is made for each of the display modes in which a display device is used. The display device may be connected to variety of signal sources that may vary in the scanning line number, the deflection frequency, and the specific aspect ratio. The term "display mode" as used herein means a specific combination of a specific scanning line number, a specific deflection frequency and a specific aspect ratio.
First, the display device is set to a specific display mode and is made to display images indicating correction points, which may a crosshatch pattern CH as shown in FIG. 44. The crosshatch pattern CH consists of a plurality of parallel vertical lines and a plurality of parallel horizontal lines. The intersection of the vertical and horizontal lines are the correction points for digital convergence.
The adjustment is accomplished by the use of the control key pad 5 which may be configured as shown in FIG. 45. The control key pad 5 includes keys for effecting various controls. FIGS. 45 shows only such keys as are necessary for the explanation of the convergence adjustment. Specifically, there are shown a groups of positioning keys 5a, including keys 5au, 5ad, 5ar and 5al for moving the cursor (or the crosshatch intersection, as will later be made apparent) up, down, rightward and leftward. The cursor may be moved for specifying intersections of a crosshatch pattern on the raster, and for inputting correction values.
The control key pad 5 also includes a mode selection key 5b for selection among various modes of key input. The mode selection key enables use of the positioning keys 5a for multiple purposes. The control key pad 5 further includes an adjustment color selection key 5a for selecting the displayed color of the crosshatch patterns CH during convergence adjustment.
During the convergence adjustment, color green of the three primary colors red, green and blue is normally taken as a reference, and colors red and blue are adjusted to register with green.
When the adjustment for color red is selected by means of the adjustment color selection key 5c, the instruction is issued from the microprocessor 6 to control the crosshatch signal generator 3 to send red and green crosshatch signals SA4 to the video circuit 4 for display on the screen.
The cursor is moved by the use of the positioning keys 5a to the correction point, i.e., the intersection of the green crosshatch pattern, correction of which is desired. Then the mode is switched from the correction point selection mode to the adjustment data entry mode by pressing the mode selection key 5b. Then, the intersection of the red crosshatch pattern is moved by the use of the positioning keys 5a so as to register the red crosshatch pattern intersection with the green crosshatch pattern intersection, and the correction value is input as correction data by pressing an entry key, not shown.
The correction data is written in the correction data memory 7. Then, the mode selection key 5b is pressed for switching to the correction point selection mode, and the cursor is moved by the use of the positioning keys 5a to the crosshatch pattern intersection for which the correction is to be effected next. The mode selection key 5b is pressed for switching to the crosshatch intersection movement mode, and the positioning keys 5a are again used to move the red crosshatch pattern intersection for registration with the corresponding green crosshatch pattern intersection.
Similar operations are repeated for each of the crosshatch pattern intersections, and the correction data written in the correction data memory 7 are used later for the color misregistration correction for the color red during use of the color display device.
Next, the adjustment color selection key 5c is pressed to select the color blue, and similar operations are repeated. The correction data written in the correction data memory 7 by the correction of the blue crosshatch pattern intersections are used later for the color misregistration correction for the color blue during use of the color display device. This completes entry of correction data for one specific display mode. The same procedure is repeated for each of other display modes.
The correction data written in the correction data memory 7 are transferred to a designated storage area in the channel memory 13.
Where the display device is used in a plurality of display modes, it is necessary to store different correction data individually for each mode. A set of correction data corresponding to each display mode is therefore stored in respective storage areas of the channel memory 13. Because each display mode is called a "channel", the memory 13 is referred to as a "channel memory". The correction data for all the display modes are stored in the channel memory 13. When the display device is used, the correction data for the display mode being selected are transferred from the channel memory 13 to the correction data memory 7, from which the correction data are read on a real-time basis for interpolating the correction data for the interpolated scanning lines and controlling the currents supplied to the auxiliary deflection coils 12.
When the adjustment is complete with respect to all the display modes, the correction data for all the display modes are stored in the respective storage areas in the channel memory 13.
The misconvergence correction during use of the display device is now described. When power to the display device is turned on, or a display mode is altered from one mode to another, one of the sets of the correction data that corresponds to the selected display mode is copied from the channel memory 13 to the correction data memory 7.
The correction data is read from the correction data memory 7 in the following manner. The address generator 2 generates an address signal SA2 (an address corresponding to the position on the raster) on the basis of the synchronous signal SA1, and also generates the interpolated scanning line number SCA signal indicative of the number of the interpolated scanning line number as counted from the top of each segment defined by adjacent horizontal lines of the crosshatch pattern. FIG. 46 shows an example of the interpolated scanning line numbers for the case where the number of the scanning lines in each segment is N-1. That is, every N-th scanning line coincides with the horizontal line of the crosshatch pattern. When the address generator 2 is addressing a scanning line which is coincident with the horizontal line of the crosshatch pattern and on which correction points lie (hereinafter, referred to as "correction scanning line"), data at the correction points are sequentially read out. When the address generator 202 is addressing an interpolated scanning line on which no correction point lies, the data of the immediately preceding correction scanning line are repeatedly read out, and the correction data for these interpolated scanning lines are obtained by means of vertical interpolation using the data of the immediately preceding correction line.
The interpolation is performed by the vertical interpolator 208. If the correction values of the correction points A0 and B0 in FIG. 44 are respectively denoted by a and b, then the correction value on the interpolated scanning lines between the points A0 and B0 is given by: EQU (b-a).times.n/N+a
where N is the number of the interpolated scanning line (n-1) between the points A0 and B0, plus 1; and
n is the interpolated scanning line number as counted from the top of the segment. PA1 said device comprising: PA1 a correction data memory for storing the correction data for said correction points; PA1 an address generator for sequentially addressing respective points along said scanning lines in the order of scanning, for the purpose of producing correction data for said respective points. In synchronism with said deflection by said deflection means, and also addressing said correction data memory for the purpose of reading the correction data from said correction data memory; PA1 the addressing for the purpose of reading being such that when one of the correction scanning lines (n-th correction scanning line) is addressed, the correction data for the correction points on the correction scanning line ((n-1)-th correction scanning line) immediately preceding the first mentioned correction scanning line are read in turn; PA1 said correction data being read from said correction data at a rate 1/K times the frequency of a clock, while said address generator is addressing said correction scanning lines; PA1 a coefficient generator for producing coefficients used for the vertical interpolation; PA1 a vertical interpolator for determining the correction data for the interpolated scanning lines on the basis of said correction data from said correction data memory and said coefficients from said coefficient generator; PA1 wherein said vertical interpolator comprises: PA1 a switching circuit receiving the correction data from said correction data memory; PA1 a delay circuit for producing first delayed data identical to an output of said switching circuit but delayed by a delay period (K.times.H-1) times said clock period, with H being the number of the reading periods per horizontal scanning period, and second delayed data identical to the output of said switching circuit but delayed by a delay period (K.times.H) times said clock period; PA1 said switching circuit also receiving said first delayed data and said second delayed data, repeatedly selecting, when the address generator is addressing the correction scanning lines, said correction data for one clock period every K clock periods and the first delayed data for the remaining (K-1) clock periods every K clock periods, and selecting said second delayed data when the address generator is addressing the interpolated scanning lines; PA1 a multiplier for multiplying one of the first delayed data and the second delayed data, and data derived therefrom by the coefficient generated by said coefficient generator; and PA1 an accumulator for accumulating the output of said multiplier for an accumulation period K times said clock period; PA1 whereby the correction data of the K correction points which are vertically consecutive and vertically aligned with each other are rearranged so as to be successive along a time axis; PA1 the K successively rearranged correction data are multiplied at said multiplier by the respective coefficients from said coefficient generator; and PA1 the products of said K successively rearranged correction data and the respective coefficients are cumulatively added to produce a correction data of an interpolated scanning line. PA1 a scanning line number detector for detecting the total number of scanning lines of a signal input to said display device. In such a case, it may be so arranged that the correction data memory is adapted to store said correction data for a predetermined total number of scanning lines, predetermined correction points, and predetermined numbers of interpolated scanning lines in each of segments formed between respective pairs of adjacent correction scanning lines; said address generator is responsive to the detected total number of scanning lines for determining, if the detected total number differs from said predetermined total number, the number of interpolated scanning lines in each segment in such a manner that the correction points on the raster for the detected total number of scanning lines are at about the same positions as the correction points for said predetermined total number of scanning lines for which said correction data memory stores said correction data memory; and said coefficient generator produces coefficients suitable for the number of the interpolated scanning lines determined by said address generator. PA1 said registration error correction device comprising: PA1 a correction data memory for storing correction data for said correction points; PA1 an address generator for sequentially addressing respective points along said scanning lines in the order of scanning, for the purpose of producing correction data for said respective points, in synchronism with said deflection by said deflection means, and also addressing said correction data memory for the purpose of reading data from said correction data memory; PA1 the addressing for the purpose of reading the correction data being such that when the address generator is addressing one of the scanning lines in one of segments; PA1 the correction data are read column by column, each column consisting of K correction points vertically consecutive and vertically aligned with each other, and including correction points above and below said segment; PA1 the correction data of correction points in each column being read from top to bottom in each column; PA1 said correction data being read from said correction data at a rate of a clock, while said address generator is addressing said correction scanning lines; PA1 a coefficient generator for producing coefficients used for the vertical interpolation; PA1 a vertical interpolator for determined the correction data for the interpolated scanning lines on the basis of said correction data from said correction data memory and said coefficient generator, said vertical interpolator including: PA1 a multiplier for multiplying the correction data output from said correction data memory by the coefficient generated by said coefficient generator; and PA1 an accumulator for accumulating the output of said multiplier for an accumulation period K times said clock period; PA1 whereby the correction data of the K correction points which are vertically consecutive and vertically aligned with each other are successively produced from the correction data; PA1 the K successively produced correction data are multiplied at said multiplier by the respective coefficients from said coefficient generator; PA1 the products of said K successively produced correction data and the respective coefficients are cumulatively added to produce a correction data of an interpolated scanning line. PA1 wherein there are further provided imaginary correction scanning lines disposed above the upper edge of the raster and below the lower edge of the raster, and imaginary correction points at intersections of said imaginary correction scanning lines and said vertical lines; PA1 said registration error correction device comprising: PA1 a correction data memory for storing the correction data for said correction points and said imaginary correction points; PA1 an address generator for addressing said correction data memory for the purpose of reading correction data from said correction data memory; PA1 a coefficient generator for producing coefficients used for a vertical interpolation; PA1 said address generator also addressing said coefficient generator to cause said coefficient generator to output coefficients according to the interpolated scanning line for which the interpolation is being conducted; and PA1 a vertical interpolator for determining the correction data for the interpolated scanning lines on the basis of said correction data from said correction data memory and said coefficient generator; PA1 wherein PA1 the correction data for the interpolated scanning lines near the upper edge of the raster are determined by interpolation using the correction data of said correction points and of said imaginary correction points above the upper edge of the raster; and PA1 the correction data for the interpolated scanning lines near the lower edge of the raster are determined by interpolation using the correction data of said correction points and of said imaginary correction points below the upper edge of the raster. PA1 a scanning line number detector for detecting the total number of scanning lines of the input signal to said display unit. In such a case, it may be so arranged that said coefficient generator further comprises an interpolated scanning line number generator for allocating interpolated scanning lines to respective segments formed between respective pairs of adjacent correction scanning lines, on the basis of the detected total number of scanning lines; a coefficient address generator for generating an address signal for the coefficient data on the basis of the number of the interpolated scanning lines allocated to each of said segments; and a coefficient data generator for supplying coefficient data responsive to said address signal; and the interpolated scanning lines are allocated to the respective segments so as to minimize the shift of the correction scanning lines due to the change in the total number of the scanning lines. PA1 said correction points including a first class of correction points, called adjustment points, and a second class of correction points, called non-adjustment correction points; PA1 said method comprising the steps of: PA1 performing adjustment to correct registration error with respect to said adjustment points to obtain correction data for said adjustment points; PA1 determining, by interpolation, correction data for said non-adjustment correction points; PA1 storing the correction data for said adjustment points and said non-adjustment correction points in a correction data memory; and PA1 using the correction data for said adjustment points and said non-adjustment correction points for the interpolation of the interpolated scanning lines. PA1 said correction points including a first class of correction points, called adjustment points, and a second class of correction points, called non-adjustment correction points; PA1 said registration error correction device comprising: PA1 means for inputting the correction data for said adjustment points; PA1 means for determining, by interpolation, the correction data for the non-adjustment correction points on the basis of the correction data for the adjustment points; PA1 a correction data memory for storing the correction data for said correction points; PA1 an address generator for addressing said correction data memory for the purpose of reading correction data from said correction data memory; PA1 a coefficient generator for producing coefficients used for the vertical interpolation; PA1 said address generator also addressing said coefficient generator to cause said coefficient generator to output coefficients according to the interpolated scanning line for which the interpolation is being conducted; and PA1 a vertical interpolator for determining the correction data for the interpolated scanning lines on the basis of said correction data from said correction data memory and said coefficient generator. PA1 said registration error correction device may further comprise means for determining the {(i-1).times.(j-1)}-th-order correction function; and PA1 said means may be adapted to determine the correction data by interpolation using the said correction function. PA1 an image pickup device for picking up at least part of the image displayed on said screen including one of said adjustment points; PA1 an actuator for moving said image pickup device means so that the image pickup device picks up a different part of the image displayed on the screen as it is moved; PA1 an error detector for detecting the registration error at each adjustment point on the basis of the image signal from said pickup device; and PA1 control means for adjusting correction data responsive to the output of said error detector. PA1 wherein the correction is conducted with a selected one of a plurality of correction patterns each defined by the number of the correction points in the vertical direction, and the number of the interpolated scanning lines in each segment; PA1 said registration error correction device comprising: PA1 means for detecting the number of the scanning lines of the signal input to the display device; PA1 means for controlling the number of the correction points in the vertical direction; PA1 means for controlling the number of the interpolated scanning lines between adjacent correction points; PA1 vertical interpolation filters for calculating the correction data on the interpolated scanning lines, each of the vertical interpolation filters designed to interpolate a specific number of the interpolated scanning lines; and PA1 means for selecting one of the outputs of said digital filters according to the number of the scanning lines; PA1 the selection of the output of the vertical interpolation filters being made such that the shifts of the positions of the correction points on the raster due to the change in the number of the scanning lines are minimized. PA1 n is an integer equal to or greater than "1". PA1 said registration error correction device comprising: PA1 means for setting the number of the interpolated scanning lines for each of the segments, in accordance with the total number of the scanning lines; and PA1 means for interpolating the correction data of the interpolated scanning lines in each of the segments, according to the number of the interpolated scanning lines in said each of the segments.
The value of n is given by the scanning line number signal SCA.
The vertical interpolator 208 may be configured as shown in FIG. 47. As illustrated, it includes an input terminal 21 for receiving the output signal SA6 of the correction data memory, an input terminal 22 for receiving the address signal SA2 which is output from the address generator 202, an input terminal 23 for receiving the interpolated scanning line number SCA from the address generator 202, a timing signal generator 24 receiving the address signal SA2 (via the input terminal 22) as one input, and receiving the interpolated scanning line number SCA (via the input terminal 23) as another input, and generating various timing signals, a coefficient generator 25 receiving the interpolated scanning line number SCA via the input terminal 23, a shift register 27 receiving the signal SA6 via the input terminal 21 and controlled by the timing signal generator 24, a subtractor 28 for subtracting the signal SA6 input via the input terminal 21 from the output signal of the shift register 27, a multiplier 29 for multiplying the output signal of the subtractor 28 by the output signal of the coefficient generator 25, an adder 30 for adding the output signals of the shift register 27 and the multiplier circuit 29, and an output terminal 31 for outputting the output signal of the adder 30 to the D/A converter 9.
In operation, the correction data SA6 from the correction data memory 7 is delayed by the shift register 27 by a time period corresponding to the vertically-separated correction point interval in the vertical direction. That is, at the time when correction data "b" of the correction point B0 is read from the correction data memory 7, correction data "a" of the correction point A0 is output from the shift register 27. The correction point A0 is one correction point interval before B0 in the vertical direction. Then, the subtraction (b-a) is performed by the subtractor 28. At the same time, the coefficient generator 25 outputs coefficient n/N corresponding to the interpolated scanning line number n, responsive to the scanning line number data SCA. The multiplier 29 multiplies (b-a) by n/N to output (b-a).times.n/N, which is, then, added with "a" by the adder 30 providing EQU (b-a).times.n/N+a
to the output terminal 31.
The correction value of the interpolated scanning lines are calculated in the manner described above.
The output signal from the vertical interpolator 208 is input to the D/A converter 9 (FIG. 43), where it is converted into an analog signal.
The sequence of digital values respectively representing the correction values along an interpolated scanning line which are output from the vertical interpolator 208, or the sequence of the digital signals respectively representing the correction values along a scanning line which are output from the correction data memory 7 are sequentially supplied to the D/A converter 9. The analog signal output from the D/A converter 9 is smoothed (and hence is effectively horizontally interpolated) by means of the LPF 10, and is supplied to the output circuit 11, which supplies a corresponding correction current to the auxiliary deflection coil 12. In this way, the convergence adjustment for each scanning line is achieved.
FIG. 48 shows another example of conventional digital convergence device. As illustrated, it comprises an input terminal 101 for horizontal blanking pulses (hereinafter referred to as "H-BLK pulses") which are horizontal reference signals for the input signal, an input terminal 102 for vertical blanking pulses (hereinafter referred to as "V-BLK pulses") which are vertical reference signals for the input signal, a vertical interpolation address generator 27, a vertical address generator 28, a input terminal 109 for horizontal addresses, a correction data memory 7 for storing data for convergence correction, a vertical interpolation filter 31 for calculating correction data between correction points in the vertical direction, a D/A converter 10 for converting digital data into an analog correction signal, and an output terminal 117 for the correction signal.
FIG. 49 shows an example of a set of convergence correction points arranged on the raster in the prior art example. In the illustrated example, 20 correction points are arranged in the horizontal direction, and 15 correction points are arranged in the vertical direction. For misconvergence correction between correction points in the horizontal direction, an analog smoothing filter (LPF) is used, and for misconvergence correction between correction points in the vertical direction, a digital interpolation filter which conducts real-time interpolation is used.
FIG. 50 shows convergence correction points on the raster in a case when the number of scanning lines of an input signal is 141. As shown in FIG. 50, where are 9 scanning lines between correction points. Thus, misconvergence correction can be made for scanning lines of (9+1).times.(15-1)+1=141.
The number of scanning lines for which the misconvergence correction can be made with a high reliability in a digital convergence system is called a correctable scanning line number. To generalize, this correctable scanning line number DN is given by: EQU DN=(IV+1).times.(NV-1)+1 (1)
where IV is the number of the interpolated scanning lines, and
NV is the number of the vertical arranged correction points.
Now the operation of the device shown in FIG. 48 is described. FIG. 48 also shows a part of the device relating to one channel only.
The H-BLK pulses, which are the reference signal for the horizontal deflection, and the V-BLK pulses, which are the reference signal for the vertical deflection, are input via the input terminals 101 and 102 and supplied to the vertical interpolation address generator 27. The address generator 27 comprises a counter whose content (count value) is incremented by clocks whose period is equal to one horizontal scanning period. For example, where the number of interpolated scanning lines is nine, the counter will be a decimal counter, which counts from 0 to 9 to generate interpolated scanning line addresses 0 through 9.
Each time the count value of the decimal counter forming the vertical interpolation address generator 27 reaches its maximum, a ripple carry is produced, and is supplied to the vertical address generator 28. The vertical address generator 28 comprises a counter which is reset by the V-BLK signal and whose content (count value) is incremented each time a ripple carry signal is produced from the vertical interpolation address generator, and successively generates address signals having address values 0 through 14. The vertical address signals are supplied to the correction data memory 30.
A horizontal address generated at a circuit, not shown, is input via the input terminal 109 to the correction data memory 30.
The correction data memory 30 stores the misconvergence correction data for predetermined correction points on the raster. The misconvergence correction data are read from the correction data memory according to the above-mentioned horizontal and vertical address signals, and are input to the vertical interpolation filter 31.
The vertical interpolation filter 31 conducts real-time interpolation of the correction data between the correction points in the vertical direction. This interpolation is conducted for each correction point.
The vertically-interpolated convergence correction data are converted by the D/A converter 10 into an analog signal and smoothed by the analog LPF 11 to become a convergence correction output, which is supplied to a current amplifying means (not shown).
Now let us assume that the above-described convergence correcting device is used for a multi-scan video projector, and the number of the scanning lines of the input signal is altered due for example to switching of the input signal.
As shown in expression (1), the number of correctable scanning lines depends on the number of correction points in a vertical direction and the number of interpolated scanning lines between correction points in the vertical direction. Since the number of interpolated scanning lines is fixed in the conventional system, to cope with the increase in the number of the scanning lines of the input signal, the number of correction points in the vertical direction is increased.
FIG. 51 shows convergence correction points on the raster for a case when the number of scanning lines of the input signal has changed to 201 in the conventional digital convergence device. For example, if the number of the effective scanning lines of the input signal is 200, then the number of correction points in the vertical direction is increased from 15 to 21. As a result, according to expression (1) the number of the correctable scanning lines becomes (9+1).times.(21-1)+1=201. In this way, misconvergence correction an be accomplished with a high reliability for the period corresponding to the effective scanning lines.
In the digital convergence devices which have been described referring to FIG. 43 or FIG. 47, linear (straight-line) interpolation is made in order to correct the convergence on the interpolated scanning lines. As a result, the error from the true correction data may not be negligible, as shown in FIG. 52. This may result in uneven distribution of the scanning lines, i.e., varying density of the scanning lines.
Another problem is that the number of the correction points in the vertical direction must be altered when the number of the scanning lines of the input signal is altered. If the number of the scanning lines of the input signals to the display device vary over the range of from 200 to 2000, the number of the correction points in the vertical direction must be altered over the range of from 21 to 201. The capacity of the correction data memory 7 must be sufficient to store the correction data for the maximum number of the correction points. The capacity of the channel memory 13 must be sufficient to store the correction data for the entire range of the variation of the total number of the scanning lines. Moreover, it takes a longer time to make adjustment and obtain the correction data. Furthermore, the correction data for one input signal may not be used by simple copying to produce correction data for another input signal because of the difference in the number of the correction points, and some sort of data conversion is required.
Devices similar to the convergence device can be used for correction of raster distortion, and similar problems are encountered in the raster distortion correction. In the present specification, the expression "registration error" is used to cover body misconvergence (convergence error) and raster distortion.